Pressure-sensitive adhesive (PSA) tapes are virtually ubiquitous in the home and workplace. In one of its simplest configurations, a pressure-sensitive tape includes a backing layer and an adhesive layer attached to the backing layer. According to the Pressure-Sensitive Tape Council, pressure-sensitive adhesives are known to possess properties including the following: (1) aggressive and permanent tack, (2) adherence with no more than finger pressure, (3) sufficient ability to hold onto an adherend, and (4) sufficient cohesive strength to be removed cleanly from the adherend. Materials that have been found to function well as PSAs include polymers designed and formulated to exhibit the requisite viscoelastic properties resulting in a desired balance of tack, peel adhesion, and shear holding power. PSAs are characterized by being normally tacky at room temperature (e.g., about 20° C. to 25° C.). Materials that are merely sticky or adhere to a surface do not constitute a PSA; the term PSA encompasses materials with additional viscoelastic properties.
Acrylic-based pressure-sensitive adhesives have been widely used. These pressure-sensitive adhesive compositions can be organic solvent-containing or organic solvent-free. PSA compositions containing organic solvents, while currently dominant in the marketplace are decreasing in importance due to the associated problems of pollution and high energy consumption associated with the use of organic solvents. That is, the adhesive industry is increasingly focused on the organic solvent free-type pressure-sensitive adhesives. PSA compositions that are organic solvent-free can be classified as emulsion type adhesives, hot melt type adhesives, or ultraviolet cured type adhesives.
With increased use of solvent-free PSA compositions, especially in the industrial fields, the performance requirements, especially for adhesion, have increased. The traditional paths for effectively enhancing the adhesion of acrylic-based PSA include adding a plasticizer or a traditional tackifier into the formulation. Plasticizers have been less commonly used than tackifiers due to process limitations, raw material issues, and stability issues. Presently, the common tackifiers used are mainly rosin resins, terpene resins, petroleum resins, aromatic resins, and the like. While these common tackifiers have been widely used, they typically have not been used for pressure-sensitive adhesives cured by ultraviolet radiation. That is, these common tackifiers tend to absorb a significant amount of ultraviolet radiation and tend to substantively inhibit the polymerization reaction, which in turn can adversely impact the performance of the resulting PSA. For example, phase separation can occur between the adhesive and tackifier and poor cohesion force can result from the low molecular weight and low percent curing of adhesive polymer.
Many attempts have been made seeking tackifiers that are suitable for use in acrylic PSA compositions that are cured by ultraviolet radiation. The tackifiers used typically have been hydrogenated rosin resins or aromatics synthetic resin but these tackifiers have considerable limitation such as high cost and limited ability to enhance the performance of the pressure-sensitive adhesives.